1. Technical Field
The present invention relates to a pump impeller suitable for use in a submersible pump for sewage treatment, and more particularly to an impeller capable of effectively removing air trapped in a region on a rear-surface side of the impeller. The present invention also relates to a submersible pump having such impeller.
2. Background Art
Conventionally, there is a submersible pump for use in sewage treatment installed in a manhole. Such a submersible pump for sewage treatment is installed with its suction pipe located in a depression which is slightly below a floor surface in the manhole, as shown in, for example, a patent document 1. In such a submersible pump, if a water level in the manhole is not enough, air pocket (i.e., trapped air) is formed on an inner surface of a pump chamber which houses an impeller therein or on a rear-surface side (i.e., an upper-surface side) of the impeller. This air pocket can be a cause of idling of the pump. Moreover, the air pocket may prevent sufficient supply of liquid (i.e., the sewage) to a mechanical seal of the pump, thus causing insufficient lubrication. In order to prevent such problems, the conventional submersible pump has an air vent valve near a ceiling of the pump chamber. This air vent valve can remove the air pocket remaining on the inner surface of the pump chamber or on the rear-surface side of the impeller.
The patent document 1 discloses, as one example of conventional submersible pump, a vortex type having a relatively flat main shroud of the impeller (the shroud is a plate that covers the rear-surface side). In this type of submersible pump, even if the air pocket is formed on the main shroud of the impeller, such air pocket can be removed sufficiently through the above-described air vent valve.
As another example of conventional submersible pump, there is a submersible pump having a non-clogging type impeller, as shown in a patent document 2. This non-clogging type impeller has a single vane with a fluid passage formed in its approximately cylindrical body. The fluid passage has a vortex shape as viewed from an axial direction of the impeller. In order to prevent the pump from being clogged with foreign substances when pumping the sewage, the fluid passage has a cross section with a substantially constant dimension such that the foreign substances are less likely to be caught in the impeller.